Hello David
The syntax for the copy command when using tftp is the following:
copy tftp:[[//location]/directory]/filename.
The “location” portion of the alias according to Cisco documentation is either a domain name or an IP address. I tried labbing this up and found that you cannot use “l0” for example to refer to the loopback interface in such an interaction. I was using the CSR1000v version 17.03.06 in Cisco’s CML.
You can however use an IP address, and in that way specify the address of the loopback interface you want to use. That assumes however that a TFTP server is running on the local device. In such a case, you would use a command similar to the following:
tftp://[TFTP_SERVER_IP]/ios-file flash:
Now what the ip tftp source-interface command does is it sets the source IP address of TFTP requests to be the IP address of the specified interface. If this command is not issued, then the exit interface based on the routing table to reach the intended destination is used as the source/destination for that interaction. By using this command you can achieve a predictable source address, since it won’t be changing based on destination. Thus, any TFTP interactions will always take place with the same local address.
You can find more info about the copy command and the related TFTP syntax at the following Cisco documentation:
I hope this has been helpful!
Laz